Discovery of the origin of fast solar wind

A team of researchers analyzing data from NASA's Parker Solar Probe has discovered the origin of fast solar wind
An illustration of NASA’s Parker Solar Probe

NASA Parker Solar Probe has flown close enough to the Sun to observe in great detail the origin of fast solar wind from “coronal holes” in the solar atmosphere. With the data collected, scientists would be able to better predict solar storms that pose a threat to our planet. In addition to the phenomenon of auroras, these events can also disrupt communications, energy infrastructure, and pose a threat to satellites and spacecraft.

Probe has traced the solar wind back to its source as reported in a study (ref.). The flow of charged particles is lost as they leave the outer atmosphere of the solar corona, before reaching Earth as a relatively uniform stream.

Probe has analyzed that the high-energy particle flows that make up the solar wind correspond to the so-called “supergranulation flows” within the coronal holes. This discovery has identified these regions as the source of the “fast” solar wind, which is seen above the poles of the Sun and can reach speeds of up to 2.7 million km/h.

Coronal Holes

Coronal holes form in areas where magnetic field lines emerge from the surface of the Sun but do not return. This creates open field lines that extend to fill the space around the star. During the quiet periods of the Sun’s 11-year activity cycle, coronal holes are located at the Sun’s poles. Therefore, the solar wind emerging from coronal holes is usually not directed toward Earth.

When solar activity increases, the magnetic field reverses polarity, coronal holes become more widespread, and these powerful streams of charged particles can also be directed towards our planet. These new findings could help in predicting potentially disruptive solar storms.

“Winds carry a lot of information from the Sun to Earth. Understanding the mechanism behind solar wind is important for practical reasons on Earth” said James Drake in a statement. “This will affect our ability to understand how the Sun releases energy and drives geomagnetic storms, which pose a threat to our communication networks”.

Fast Solar Wind

Coronal holes function like a shower, spraying jets of charged particles from equidistant “bright points” where magnetic fields extend from the surface of the Sun. This creates funnels that can be about 29,000 kilometers wide. The team believes that when magnetic fields with opposite directions intersect in these funnels, the magnetic field lines break and then reconnect. This process, called magnetic reconnection, is responsible for the expulsion of charged particles that we see as solar wind.

But why are the speeds of some of the observed particles up to 10 times higher than the average solar wind? Scientists think that this is only possible thanks to magnetic reconnection. Such speeds are not possible for particles simply navigating through plasma. “The photosphere is covered with convection cells, like in a boiling pot of water, and the convection flow on a larger scale is called supergranulation” said Stuart Bale, co-author of the research.

“The big conclusion is that it is magnetic reconnection within these funnel structures that provides the source of energy for the fast solar win,” he reiterated. “It doesn’t just come from all over a coronal hole, but it is structured underneath within coronal hole funnels to these supergranulation cells. It comes from these small bundles of magnetic energy that are associated with convection flows. Our results, we think, are strong evidence that reconnection is the driving force behind it.”

Analysis of the Parker Solar Probe

The Parker Solar Probe was launched on August 12, 2018. As of March 17, 2023, the spacecraft had completed 15 close approaches to the Sun. The passes are at approximately 6.1 million km with a speed of 587,000 km/h. “Once you get below that altitude, about 11 million to 13 million miles or so, there’s much less evolution of the solar wind. The flow is more structured, and you see more imprints of what was on the Sun” said Bale.

In 2021, the spacecraft passed about 8.4 million km from the solar surface and encountered jets of material. At that time, the team was uncertain whether those charged particles were accelerated by magnetic reconnection or by hot plasma waves. “Our interpretation is that these outflows of reconnection excite Alfvén waves as they propagate” Bale said. “This is a well-known observation also from Earth’s magnetotail, where you have similar types of processes”.

New data from the Parker Solar Probe may come when the probe passes at around 6.4 million km. These future close approaches could help the team confirm their theory. However, this could be complicated by the fact that the Sun is about to enter the solar maximum, a period of chaotic and intense activity.

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